Difference between revisions of "Assignment 8, Part 2: fabricate a microfluidic device"

Revision as of 21:43, 23 October 2018

Overview

In this part of the assignment, you will make a microfluidic device out of double-sided tape sandwiched between a glass coverslip and a slab of PDMS. This procedure is incredibly simple, as far as microfluidics goes, and was developed in Paul Blainey's lab to image the motion of transport molecules along DNA ^[1].

PDMS is a silicone elastomer made by mixing together a viscous liquid base with a crossliking agent. Once mixed, the material will harden into a solid a rubber-like material. The hardening process is accelerated by heating to 60°C (taking hours rather than several days). The resulting material will be optically clear, non-toxic, and chemically inert. Researchers typically use PDMS for microfluidics because they can use it to cast very small sharp features (down to ~1 micron), and they can covalently bond the PDMS to a glass coverslip, creating a sealed device that is readily compatible with most types of microscopy.

Since our experiment does not require extremely small features, we will simply cut the Y-shaped channel out of a piece of double-sided tape using a vinyl cutter, rather than casting PDMS over a mold. We're still going to take advantage of PDMS, though. First, it will provide an optically clear structure for our device, and second, because it is somewhat soft and flexible, we can easily punch holes in it (which is hard to do with something like glass) in order to connect inlet and outlet tubing to our flow channel.

Cast a slab of PDMS

This protocol has two 30-minute wait times. If you'd like to keep making progress while you wait, Part 2 of this assignment can be completed in parallel.

1. If not already on, turn on the oven to 60°C.
2. Using the scale, measure 63g of the Sylgard™ 184 Silicone Elastomer base, plus 7 g of the curing agent in a paper cup.
3. Mix the elastomer base and curing agent REALLY WELL using a plastic stirrer.
4. Pour the mixture into a square petri dish.
5. Degas the PDMS for 30 minutes using the vacuum dessicator in the fume hood.
6. After 30 minutes, turn off the vacuum to the desiccator, and slowly vent the chamber by turning the red T.
7. Remove ALL remaining bubbles with a pipette tip.
8. Place the petri dish in the 60°C oven for 30 minutes. Typically PDMS is cured for 2 hours, but 30 minutes will provide sufficient hardening for us.

You may store the cured PDMS in the petri dish as is if you do not want to complete the next step in the procedure right away.

Assemble your device

Microfluidic device assembly.

1. Remove a rectangle of pre-cut double-sided tape from the large sheet. (Do not yet remove the clear backing!)
2. Cut out a slab of PDMS roughly the same size as the tape, or slightly larger. Leave it in the dish until you are ready to use it to prevent dust accumulation.
3. Using tweezers, remove the internal Y-shape from the cut tape. Make sure to remove the green part as well as both sides of the clear Y-shaped backing.
4. Use tweezers to peel off the clear backing from ONE side of the cut tape. Remove the PDMS slab from the petri dish, and stick the flat side (bottom) onto the green tape. Press everywhere to ensure a full seal.
5. Use a 0.5mm biopsy punch to make holes for the inlet and outlet tubing. After inserting the biopsy punch, make sure to remove the small core that it creates before extracting the punch from the PDMS.
6. Peel off the remaining clear backing from the tape using tweezers, and seal the flow channel to a 22x40 mm coverslip. Apply pressure to the PDMS, rather than the coverslip, to prevent cracking.
7. To assemble the inlet and outlet tubing, connect one steel tube to a ~2" length of tygon tubing. For the two inlet sections of tubing, connect a second steel tube to the opposite end.
8. Insert the inlet and outlet tubing into the punched holes in the PDMS.

Your device is now assembled!

References

1. ↑ K. Xiong and P. C. Blainey, “A Simple, Robust, and High Throughput Single Molecule Flow Stretching Assay Implementation for Studying Transport of Molecules Along DNA,” J. Vis. Exp., no. 128, pp. 1–7, 2017

Navigation

• Overview
• Part 1: feedback systems
• Part 2: fabricate a microfluidic device
• Part 3: add flow control and test your device

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